Lead frames are each formed of a metal plate in which fine patterns such as inner leads and outer leads are processed. After mounting a semiconductor chip on an inner portion of the lead frame, terminals of the semiconductor chip and the inner leads are electrically connected to each other.
Recently, there has been an even stronger demand for a higher packing density and higher integration of semiconductor chips. Corresponding to such a demand, lead frames for mounting semiconductor chips thereon have also been developed which are of fine shapes and highly accurate dimensions. In particular, great weight has been given to increasing the number of pins per lead frame. With an increase in the number of pins per lead frame, a processing technique is required which can form distal end portions of inner leads to be smaller in pitch and finer in shape.
Recent techniques for forming such lead frames or the like from metal plates have been primarily practiced by pressing or etching. By those processing techniques, lead frames can be easily manufactured with high efficiency. However, any of etching and pressing can only form gaps comparable to the thickness of a metal plate used, and cannot satisfactorily form gaps narrower and finer than the plate thickness.
Meanwhile, there is proposed a method of fabricating lead frames by a laser beam (hereinafter referred to also as laser cutting). This method makes it possible to form finer gaps beyond limitations in etching and pressing. Specifically, a laser beam used in the above method is generally circular in section, but because the laser beam can be condensed into a very small diameter, it is possible to achieve very fine processing. Regardless of the thickness of material plates employed, therefore, the pitch between inner leads can be made much smaller. (Hereinafter, the above method will be referred to as laser cutting using a circular beam).
Besides the laser cutting using a circular beam, there is also known a processing technique that employs a laser beam having an elongate elliptic section, as disclosed in JP, A, 1-306088 or JP, A, 62-93095, and a processing technique that employs a laser beam having an elongate section, i.e., a slab laser or the like, as reported in Electric Society of Japan, Photon Device Meeting (August, 1989).
Further, as another prior art technique using laser cutting, JP, A, 2-247089 discloses a method in which laser cutting is combined with pressing or etching. With this method, fine portions having a narrow pitch, such as distal end portions of inner leads of the lead frame, are formed by a laser beam, whereas relatively large portions not having a narrow pitch, such as outer leads, are formed by pressing or etching.